MCP73123-22SSLASHMF datasheet(16/28 Pages) MICROCHIP

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MCP73123/223

DS22191E-page 16

 2009-2013 Microchip Technology Inc.

6.1

Application Circuit Design

Due to the low efficiency of linear charging, the most

important factors are thermal design and cost, which

are a direct function of the input voltage, output current

and thermal impedance between the battery charger

and the ambient cooling air. The worst-case situation is

when

the

device

has

transitioned

from

the

Preconditioning mode to the Constant Current mode. In

this situation, the battery charger has to dissipate the

maximum power. A trade-off must be made between

the charge current, cost, and thermal requirements of

the charger.

6.1.1

COMPONENT SELECTION

Selection of the external components in Figure 6-1 is

crucial to the integrity and reliability of the charging

system. The following discussion is intended as a guide

for the component selection process.

6.1.1.1

Charge Current

The recommended fast charge current should be

obtained from the battery manufacturer. For

example, a 1000 mAh battery pack with 2C preferred

fast charge current has a charge current of 1000 mA.

Charging at this rate provides the shortest charge cycle

times

without

degradation

the

battery

pack

performance or life.

6.1.1.2

Thermal Considerations

The worst-case power dissipation in the battery

charger occurs when the input voltage is at the

maximum and the device has transitioned from the

Preconditioning mode to the Constant Current mode.

In this case, the power dissipation is calculated using

Equation 6-1.

EQUATION 6-1:

Power dissipation with a 5V, ±10% input voltage

source, 500 mA ±10% and preconditioning threshold

voltage at 2V is calculated using Equation 6-2.

EQUATION 6-2:

This power dissipation with the battery charger in the

DFN-10

package

will

raise

the

temperature

approximately 83

C above room temperature.

6.1.1.3

External Capacitors

The MCP73123/223 is stable with or without a battery

load. In order to maintain good AC stability in the

Constant Voltage mode, a minimum capacitance of

1 µF is recommended to bypass the VBAT pin to VSS.

This capacitance provides compensation when there is

battery

load.

addition,

the

battery

and

interconnections appear inductive at high frequencies.

These elements are in the control feedback loop during

Constant

Voltage

mode.

Therefore,

the

bypass

capacitance may be necessary to compensate for the

inductive nature of the battery pack.

A minimum of 1 µF is recommended for the output

capacitor, and a minimum of 1 µF is recommended for

the input capacitor in typical applications.

TABLE 6-1:

MLCC CAPACITOR EXAMPLE

Virtually any good quality output filter capacitor can be

used,

independent

the

capacitor’s

minimum

Effective Series Resistance (ESR) value. The actual

value of the capacitor (and its associated ESR)

depends on the output load current. A 1 µF ceramic,

tantalum, or aluminum electrolytic capacitor at the

output is usually sufficient to ensure stability.

6.1.1.4

Reverse-Blocking Protection

The MCP73123/223 provides protection from a faulted

or shorted input. Without the protection, a faulted or

shorted input would discharge the battery pack through

the body diode of the internal pass transistor.

Note:

Please consult with your battery supplier

or refer to the battery data sheet for the

preferred charge rate.

PowerDissipation

DDMAX

PTHMIN

–

 I

REGMAX



Where:

VDDMAX = the maximum input voltage

IREGMAX = the maximum fast charge current

VPTHMIN = the minimum transition threshold

voltage

MLCC

Capacitors

Temperature

Range

Tolerance

X7R

-55

C to +125C

±15%

X5R

-55

C to +85C

±15%

Po werDissipa tio n

5.5V

2 V

–

 550mA



1.925 W

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